Structural and gelling properties of whey proteins influenced by various acids: Experimental and computational approaches

Five different acids, i.e., hydrochloric acid (HCl), acetic acid, lactic acid, γ-aminobutyric acid (GABA) and taurine were applied to investigated their effects on heat-induced gelation properties of whey protein isolate (WPI) at pH = 5.5. All WPI gels were found to have self-similar structures with the fractal dimension, df = 2.40. Those denatured WPI formed strong physical gels that were susceptible to shear and strain. Of all the acids, HCl, GABA and taurine significantly strengthened WPI gels. Moreover, denser packing of protein aggregates was observed from the gels with GABA and taurine. Besides experimental approaches, molecular dynamics (MD) simulations were also applied to investigate the effects of various acids on denaturation of β-lactoglobulin (β-LG). GABA and taurine were found to protect β-LG from denaturation and delayed the molecular heat-induced unfolding. Moreover, presence of HCl, GABA and taurine conserved the rigidity of unfolded β-LG and localized the residue –SH121, which was believed to result in promoted reaction-limited clustering aggregations (RLCA) of β-LG molecules and strengthened WPI gels. Further studies suggested that there was no strong binding or aggregation between cations (Na+) or anions/zwitterions and β-LG molecule. Therefore, the effects from various acids (anions/zwitterions) on β-LG and WPI were supposed to result from long-distanced electrostatic interactions and/or their effects on the protein hydration shells.